The present invention, in some embodiments thereof, relates to the detection of a respiratory signal and, more particularly, but not exclusively, to the detection of a respiratory signal by analysis of a plethysmograph signal, e.g., a photoplethysmograph signal.
An individual's health and fitness level may be determined by measuring his or her breathing patterns during respiration. In turn, respiration patterns also influence the fitness level and health of the individual. Two components of the measured respiration patterns are respiration rate and respiration depth. Respiration rate is a measure of the number of breaths taken per unit time, typically measured in breaths per minute. Respiration depth is a measure of the extent to which an individual's lungs expand and contract.
The respiration of an individual or patient may be monitored for a variety of reasons. For example, knowledge about a patient's respiration may assist a physician in assessing the patient's stability during surgery and recovery after surgery. Another rapidly growing field in which information about an individual's respiration may be of value is the field of sleep therapy.
Known in the art are monitoring systems that monitor the respiration frequency of the patient, record the respiration curve and indicate respiration irregularities such as apnea. Various such monitoring systems have been heretofore suggested and/or utilized in a variety of settings, and have included devices utilizing impedance or inductance plethysmography, aural monitoring, EMG or ECG monitoring, strain gauges and the like (see, e.g., U.S. Pat. Nos. 4,815,473, 5,083,560, 6,142,953, 6,377,845, 7,177,686 and 7,351,208).
Photoplethysmographic systems are widely used for monitoring the oxygen status of blood. A photoplethysmograph system typically includes a sensor which is typically attached to an adult patient's finger or an infant patient's foot. The sensor typically includes red and infrared (IR) light-emitting diodes (LEDs) and a photodiode detector. Light emitted from the LEDs passes through the tissue (finger or foot). The photodiode is positioned opposite to the LEDs so as to detect the light as it emerges from the tissue. Oxygen saturation is determined by computing the differential absorption by arterial blood of the two wavelengths emitted by the sensor.
Other than pure oximetry, the photoplethysmograph signal has been used for sleep monitoring and diagnosis of periodic breathing including detection of sleep apnea events, detection of cardiac arrhythmias and heart failure and prognosis of heart disease (see, e.g., International Patent Publication No. WO2009/118737, and U.S. Published Application Nos. 20070213620, 20070213621, 20070213622 and 20070213624, the contents of which are hereby incorporated by reference).